The present invention relates to a switch, and more particularly to an electromagnetic switch.
The electromagnetic switch plays an important role in the modern industrial power supply control and is extensively used in various factories, power panels in the stock room or kinds of ships, elevators, escalators, winches, conveying belts, power switches of working machines or machinery, large equipments, or panels of generating or transforming plants. The working principle of the electromagnetic switch is to mutually contact or release contactors thereon by building a magnetic field around the coil, thereof, to thus energize or de-energize the switch. The current flow depends on the DC type and the AC type in which the former provides a stable power supply but is unsuitable for industrial distribution. The latter can provide a large electrical power, but the contactors thereof have an unstable contact. In detail, since the DC type has a DC working current, the built magnetic field is stable as schematically shown in FIG. 1. Since the industry generally requires an AC power source and, for controlling a relatively small DC power source of 12 V, 24 V or 48 V, the DC electromagnetic switch is not suitable for large power panels in various industries. The AC electromagnetic switch, however, suffers from the following disadvantages:
1) Since the coil of the AC electromagnetic switch has an AC power source normally of 110 V or 220 V (totally amounting to about 90% out of all power sources), the exciting field resulted by the coil will have an alternating magnetic attraction following the alternating change of the AC voltage of the power source. In order to smooth the strength of the alternatingly changing magnetic field, the magnetic core 1 made of silicon steel in the AC electromagnetic switch as shown in FIG. 2 incorporates therewith short-circuited copper rings 4, which attempt to balance and stabilize lines 2 of magnetization. The magnetic attraction is thus produced, however, not so stable as that produced by the magnetic field of a DC electromagnetic switch.
2) Incorporating copper rings 4 to magnetic core 1 will increase the copper loss in the switch. The copper loss transforms into the heat not only represents an energy loss but also reduces the life period of the switch.
3) As shown in FIG. 3, when the coil 3 is flowing therethrough a current and the magnetic attraction overcomes the spring force exerted by the spring 5, the increasing magnetic attraction will eventually engages the electromagnetic switch to close contactors 6 thereon. Since the clearance between the upper and lower magnetic cores 1 is nearly not in existence now, the magnetic reluctance of the magnetic circuit of lines 2 is reduced, so that a small coil maintaining current will be enough to produce a magnetic attraction capable of overcoming the spring force of spring 5 to keep the switch in a holding state. Since the coil of the conventional electromagnetic switch is directly power-supplied by the power source 7 as shown in FIG. 4, the coil cannot be only provided with a maintaining or reduced current after the switch is engaged, which is not energy-effective.
4) The magnetic attraction exerted by coil 3 will increase immediately after coil 3 is switched on. As shown in FIG. 5, when there is no working voltage supplied to coil 3, contactors 6 are in an open state and thus the switch is not in operation. When t=a, coil 3 begins to establish a magnetic field but contactors 6 are still not closed. When t=b, magnetic attraction is approximately equal to the spring force exerted by spring 5 and thus the electromagnetic switch is in a floating state. When t=c, the working voltage is equal to the engaging voltage 8 (Ve), which means that the resulted magnetic attraction is larger than the exerted spring force so that contactors 6 are closed to engage the switch. At the time period between t=b and t=c, contactors 6 have a bouncing contact and sparks therebetween which not only damages contactors 6 but also adversely influences the power-supplied load device.
5) During the time period between t=c and t=d, the working voltage is stable and thus the switch is kept in a holding state. When t=d, the attraction of the magnetic field resulted by coil 3 is approximately equal to the exerted spring force again which means that contactors 6 will have a bouncing contact and sparks therebetween. When t=f, the working voltage is equal to the releasing voltage 9 (Vr) and the magnetic attraction can no more overcome the spring force so that contactors 6 are in an open state again to release the switch. When t=g, the magnetic field built by coil 3 vanishes into the void. Thus, in an operation cycle of the electromagnetic switch, there are two time periods during which contactors 6 will have a bouncing contact and sparks therebetween.